1. Field of the Invention
The present invention relates to a non-contact tonometer used in an eye doctor's office etc.
2. Related Background Art
In a conventional non-contact tonometer, a piston is moved in a cylinder driven by rotation of a rotary solenoid to compress the air in the cylinder so that a compressed air is blown to a cornea of an eye to be examined (i.e. a patient's eye). At the same time measurement light is projected onto the cornea from a light source and reflection light from the cornea is received by a photo sensor or the like. Then, certain deformation of the cornea is detected by detecting a peak of the output signal from the photo sensor, and the internal pressure in the interior of the cylinder is measured. The measured pressure is converted to an intraocular pressure value.
In the case that alignment is displaced due to a failure in fixation of the eye to be examined upon measurement or in the case that eyelashes overlap the measurement field, the photo sensor cannot receive a light flux normally. In that case, the output of the photo sensor is weakened and reliability of the measurement becomes low even when a peak of the deformation signal is detected.
In view of the above, in the case that the peak of the output signal of the photo sensor is lower than a predetermined reference value, the measurement result at that time is disregarded as an error or a mark indicating low reliability is annexed to the measurement result.
However, the reflectance of the cornea of the eye to be examined differs between individuals, while a certain constant value is used as the reference value or the criterion of reliability of measurement regardless of the reflectance of the cornea. Consequently, in the case that an eye having a high cornea reflectance is examined, there is a risk that the output from the photo sensor will not becomes lower than the reference value even if misalignment or the like occurs at the time of measurement and the output is weakened. In other words, there is a risk that the measurement result could be determined as normal in spite that the reliability of the result is low in reality.
On the other hand, in the case that the reflectance of a cornea is low, since the output from the photo sensor is low, there is a risk that the output from the photo sensor will not becomes higher than the reference value and the measurement will be regarded as an error or the mark indicating low reliability will be annexed, in spite that the measurement has been normally performed.